The tomographic measurement method that uses light having low temporal coherence as a probe, called “Optical Coherence Tomography” (OCT), is one type of nondestructive tomographic measurement technology used in medical and other fields (refer to Patent Literature 1). OCT uses light as a measurement probe, and therefore provides the advantage of being able to measure refractive index distribution, spectrum information and polarization information (double refractive index distribution), among others, regarding the measured sample.
General OCT is based on Michelson's interferometer, and its operating principles are explained in FIG. 8. Light emitted from the light source 63 is split into parallel beams through the collimator lens 62 and then divided into reference light and object light by the beam splitter 57. The object light is focused onto the measured sample 59 through the object lens 58 in the object arm, scattered and reflected by the measured sample, and then returned again to the object lens 58 and beam splitter 57.
On the other hand, the reference light passes the objective lens 60 in the reference arm, is reflected by the reference mirror 61, and then goes through the object lens 60 again to be returned to the beam splitter 57. After returning to the beam splitter 57, the object light and reference light together enter the focusing lens 56 and are then focused onto the optical detector 55 (a photodiode, etc.).
As for the light source of OCT, light having low temporal coherence is used (this is because these lights, when emitted from the same light source at different timings, are highly unlikely to interfere with each other). With Michelson's interferometer that uses light having low temporal coherence as a light source, interference signals manifest only when the distance of the reference arm and object arm is roughly equivalent. As a result, interference signals can be obtained as a function of differential optical path length (interferogram), by measuring the intensity of interference signals using an optical detector while changing the differential optical path length (τ) between the reference arm and object arm.
The shape of this interferogram represents the reflection coefficient distribution of the measured sample 59 in the depth direction, and the structure of the measured sample 59 in the depth direction can be obtained through a one-dimensional scan in the axial direction. In other words, OCT makes it possible to measure the structure of the measured sample 59 in the depth direction by scanning the optical path length.
In addition to scanning in the axial direction as mentioned above, a two-dimensional scan where mechanical scanning is also performed in the lateral direction provides a two-dimensional cross-section image of the measured sample 59. Various scanning apparatuses are available to perform this lateral-direction scan, such as apparatus having a structure whereby the measured sample is moved directly, a structure whereby the object is fixed and only the object lens is shifted, and a structure whereby both the measured object and object lens are fixed and the angle of the galvano mirror placed near the pupil surface of the object lens is turned.
In addition to the aforementioned OCT technology, another technology is known whereby the wavelength spectrum of light reflected by the measured object is acquired by a spectrometer and then the obtained spectral intensity distribution is converted by Fourier transformation. This technology is called “Spectrum interference Optical Coherence Tomography (also known as Fourier Domain OCT, or FD-OCT)” (refer to Patent Literature 2).
Normally with Fourier domain OCT, actual signals that have been acquired (interference signals indicating spectrum interference fringes) are converted by Fourier transformation to retrieve signals in the actual space (OCT signal space). However, acquired spectrum interference fringes are normally actual signals that have no complex information. For this reason, converting these interference fringes by Fourier transformation to acquire OCT signals will result in complex conjugated signals flipped over the axis, as well as auto-correlation signals, manifesting in a manner overlaying with the true OCT signals. These two signals are called “coherent noise” and have negative impact on OCT measurement if overlaid with the true OCT signals. To prevent this from occurring, the range of the measured object must be limited, and the measurement range is limited to a half or even less as a result.
One known traditional method to solve this problem is the phase shift method. Under the phase shift method, multiple spectrum interference fringes relating to the same measured location are acquired while the optical path length of reference light is changed, and phase information (that is, complex information) of the spectrum is restructured from the obtained spectrum interference fringes, in order to erase complex conjugated images.
FIG. 9 is a temporal diagram under the phase shift method. It shows the operation, per one scan cycle, of the galvano mirror used for lateral-direction scanning against time t. (i) shows the operating status trigger of the CCD camera. Here, the CCD camera starts acquiring images at time t0. (ii) indicates the scan position (position in the lateral direction) that changes in discontinuous steps during one scan cycle of the galvano mirror. (iii) indicates the position of the reference mirror moved by means of a piezo element, where measurement is performed multiple times (or three times in this diagram) at each scan step position of the galvano mirror.
Patent Literature 1: Japanese Patent Laid-open No. 2002-310897
Patent Literature 2: Japanese Patent Laid-open No. Hei 11-325849